Prestressed composite pile



Aug. 8, 1944.

M. M. UPSON` PRE-STRESSED COMPOSITE PILE Filed June` 20', 1942 @f//VFORCED COA/CRETE P/L E INVENToR.- 44AM/fa /M Z/Qso/v ATTORNEY Patented Aug. 8, 1944 PRESTRESSED COMPOSITE PILE Maxwell M. Upson, Englewood, N. J., assgnor to Raymond Concrete Pile Company, New York, N. Y., a corporation of New Jersey Application June 20, 1942, Serial No. 447,872

(Cl. (i1-56) 3 Claims.

This invention pertains to improvements in concrete piles for dock and pier work in which the concrete must be protected from the deteriorating chemical action of sea water and the destruction wrought by oxidation of the reinforcing steel.

As is well known, when piles are subjected to the rise and fall of the tide the portions of the piles exposed alternately to air and water are subject to much more rapid disintegration and deterioration than are those portions which are continuously submerged. Many methods have been tried in efforts to meet this condition, none of which has been in all ways satisfactory. The enlarging of the cross-section of the pile and placing of the steel further inward so that it is far removed from the surface results only in increasing the surface cracks on the pile and in consequence the porosity of the concrete protecting the steel. Furthermore, large piles are extremely heavy and expensive. A pile twentyfour inches in diameter and one hundred feet long weighs in the neighborhood of 35 tons. This requires special and very expensive equipment for handling and driving, which is expensive not only in cost but also costly to transport.

One of the principal causes of disintegration of concrete piles in the area exposed between low and high water has to do with shrinkage of the concrete. Pre-cast piles in order to be handled must have a substantial amount of longitudinal steel in order to provide sufficient strength to take care of the handling stresses. As is well known, concrete shrinks when it sets. The steel does not shorten during the period of setting and in consequence the steel tends to hold the interior concrete from moving. As a result the surface of the concrete develops an innite number of very minute cracks. These cracksl are usually extremely small and frequently can not be seen without a magnifying glass. However, they provide openings which admit water and air into contact with the steel reinforcing. This causes oxidation. 4Since ferrie oxide occupies more than twice the volume of metallic steel it naturally disrupts the concrete and destroys the strength of the unit.

In addition, sea water in many cases carries sulphates which combine with any free calcium hydroxide in the concrete to form calcium sulphate which in turn reacts with hydrated calcium aluminates to form calcium sulfo-aluminate. This compound swells and disintegrates the surface of the concrete. It is, therefore, evident that any method that can be used to make that section of the pile that is exposed above low water impenetrable and free from the consequences of contraction cracks is of importance.

The formation of cracks during setting may be prevented by pre-stressing thesteel and pouring the concrete around the steel while it is in tension. In the practice of this process the steel usually is of comparatively small diameter and of very high elastic limit. It is pre-stressed almost to its elastic limit during the pouring of the concrete. As the concrete sets the tension is released and the steel shortens. The amount of shrinkage of the concrete is not sufficient to take all the tension out of the steel. In consequence the steel continues its tension perpetually and keeps the concrete under continuous compression, preventing surface cracks and at the same time providing a denser and more compact concrete.

This method of pre-stressing concrete is known. However, it has been impracticable and not commercial to build forms of suflicient strength to permit the pre-stressingr of the steel during the period of pouring the concrete. Hundreds of tons of resistance may be necessary in order properly to pre-stress the steel. I therefore contemplate forming a pile having only the upper portion prestressed, the rest being reinforced in the conventional manner. This invention covers my improved pile as an article of manufacture as distinguished from the method of making it.

In the drawing, which is a sectional elevation broken away at its lower portion, I have shown a reinforced concrete pile illustrative of one form of my invention. It will be noted that the steel reinforcement above the low tide level, and up to the high tide level, is pre-stressed, while the steel reinforcement below the low tide level is not prestressed.

As stated, this invention covers my improved pile as an .article of manufacture as distinguished from the particular method of making it. My novel pile may be constructed by use of a slightly modified form of the apparatus shown in the United States Patent No. 2,080,074, patented May 11, 1937, on application led by Freyssinet et al., as will now be apparent to those of average skill in the art, after having read the foregoing disclosure of my invention.

I claim:

1. A concrete pile having steel reinforcement throughout its length, the reinforcement in a pre-selected portion of said pile being pre-stressed as compared with the reinforcement elsewhere in said pile.

level, said pile having longitudinal steel reinforcements throughout its length, the steel in only the said upper part of the pile being longitudinally pre-stressed, and the steel in the said 5 lower part being substantially free from longitudinal tension.

MAXWELL M. UPSON. 

